Hemoglobin measurement is recognized as one of the most useful procedures in clinical medicine, particularly for the detection of anemia. But such measurements have not generally been carried out with a high degree of accuracy. General purpose laboratory spectrophotometers are accurate, but they are unsuitable for general clinical use because they are expensive and require elaborate set-up procedures. Other photometers are available which are dedicated exclusively to hemoglobin determination. These are relatively inexpensive and easy to use, and are often employed in clinical settings such as doctor's offices and hospitals. For maximum convenience such instruments are usually small, portable and battery-powered. But these clinical instruments are not as accurate as might be desired.
Blood turbidity is another measurement which has diagnostic significance. Such measurements can also benefit from an improvement in the accuracy of clinical instruments.
There are a lot of separate factors, each of which makes some partial contribution of its own to the inaccuracy of such instruments. The batteries (power cells) which supply power for portable instruments tend to change their output over time, as they become exhausted; and this causes changes in the intensity of the illumination, which is an important source of inaccuracy. Any reduction in current drain therefore contributes to accuracy by extending battery life. Use of solid-state circuitry exclusively is one way to reduce current drain. Light-emitting diodes (L.E.D.'s), for example, consume less current than incandescent light sources. Solid state photodetectors are also superior in this respect to photomultipliers and other vacuum tube devices. Even within the field of solid-state circuitry, some devices require less power than others; thus a reflective (liquid crystal) type of digital display device is preferable to a galvanometer read-out or even to a digital display of the L.E.D. type.
It is also important to regulate battery output as closely as possible in order to minimize inaccuracies due to supply changes. A single Zener diode cannot do as good a job as a sophisticated integrated circuit regulator which contains the equivalent of many discrete components.
Spectral purity is a major determinant of spectrophotometric accuracy where hemoglobin determinations are concerned. Such determinations are commonly based on absorbance of green light at a wavelength of 540 nanometers, which obeys the Beer-Lambert law. When the illumination employed contains other spectral components, this law becomes a less accurate description of the absorbance function, and an instrument based on it is subject to some degree of inherent inaccuracy. In the past the light sources used in clinical hemoglobinometers have had spectral characteristics spanning the green-yellow range, and yellow-blocking filters have been used to discard the undesired yellow portion of the output. As a result, the usable light output has been severely reduced, and the signal-to-noise ratio adversely affected. It is desirable that the peak output of the unfiltered light source approached as closely as possible to the 540 nm. wavelength of interest. Furthermore, if the light source has a significant output in other regions of the spectrum, it is desirable that filters be used which cut off transmission as close as possible to the desired wavelength.